Interferons (IFNs) are cytokines that have anti-proliferative, anti-viral and immunoenhancing biological properties. IFNs induce the transcription of early responsive genes which are regulated by the activation of the Jak/Stat pathway. In T cells, incubation of peripheral blood lymphocytes or Jurkat cells (a human derived T-cell leukemia) with IFNalpha induces T cell receptor (TCR) signaling components: protein tyrosine phosphatase CD45 and protein tyrosine kinases Lck and ZAP-70 to rapidly associate with the alpha chain of the IFN receptor (IFNalpha R1). The requirement for these TCR signaling enzymes in IFNalpha-mediated activation of the JAK/Stat pathway, anti-proliferative and anti-viral effects were studied in Jurkat cells lacking these proteins. INFalpha-stimulation of the Jak/Stat pathway and anti-viral activity were intact in cells deficient in CD45, Lck or ZAP-70 whereas the anti-proliferative effect was affected but could be restored when cells were reconstituted with these proteins. These findings demonstrate that IFNalpha exerts in anti-proliferative effect by activating a novel signaling pathway in T cells which utilizes components of the TCR signaling cascade. The hypothesis to be tested will be that besides the Jak/Stat pathway, IFNalpha activates a different signaling cascade in T cells that requires protein tyrosine phosphatases and protein tyrosine kinases used by the TCR to drive the anti-proliferative activity. In order to prove this hypothesis, cDNA constructs encoding CD45, ZAP-70 or Lck that contain point mutations or deletions in structural motifs which are essential for the function of these proteins will be expressed in Jurkat cells that lack these proteins respectively. This will allow us to identify domains in these proteins that are required for binding to IFNalpha R1 and determine the biological significance of these domains to inhibit the ability IFNalpha to induce an anti-proliferative effect in cells expressing these constructs. The significance of these signaling proteins in non-transformed primary T cells will be examined using thymocytes from genetically engineered mice which are lacking these proteins. In addition, requirement for the Jak/Stat pathway to participate in the activation of this signaling network will be examined using cell lines that are deficient in Jak1, tyk2, Stat 1 and Stat 2 proteins.